Microsoft also contested Panda's claim that it had reported a problem. "Microsoft is currently aware of and is reviewing reports regarding public claims of attacks on IIS Web servers," said Bill Sisk, a communications manager who works in the Microsoft Security Response Center. "While we have not been contacted directly regarding these reports, we will continue to monitor all reports either publically shared or responsibly disclosed and investigate once sufficient details are provided."

Although it may not be clear how attackers are compromising such large numbers of Web sites, what happens after a site is infected is well-understood, researchers have said. When a visitor reaches one of the hacked sites, malicious JavaScript loads an IFRAME from a malware-hosting server; the IFRAME redirects the browser to a different page, also hosted on the hacker's server.

Next, a multiple-strike attack kit is downloaded to the visitor's PC. The kit tries eight different exploits, and if it finds one that works, hijacks the system.

These kinds of attacks, said Sherstobitoff, essentially make the idea of a "trusted site" moot. "You used to know that if you walked down the dark streets of the Web, you would be infected. Today, you really can't tell what the dark streets are."

The hacker strategy, of course, is to leverage that uncertainty. "This is getting really bad," Sherstobitoff said.

It's so bad, in fact, that while security companies urged Web site administrators to check their server logs for evidence of a compromise, and told corporate security staffs to block several malware-hosting sites at their companies' perimeters, they didn't have much useful advice for end-users.

"Users should be extremely wary when visiting sites, even those typically trusted," was about all Symantec could come up with in an alert to customers of its DeepSight threat notification service.

Disabling JavaScript can also protect against such attacks, Symantec added. Users, however, are often reluctant to switch off JavaScript because without it, many sites are crippled or won't display properly.